Crews try to tow Oracle's AC72 back to San Francisco after the yacht flipped during training. The wing - the device at the center of the boat that catches the wind - was badly damaged.

Photo: Brant Ward, The Chronicle

Crews try to tow Oracle's AC72 back to San Francisco after the...

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After Oracle's AC72 flipped during training, crews accompany the racing craft as currents carry it toward the ocean.

Photo: Brant Ward, The Chronicle

After Oracle's AC72 flipped during training, crews accompany the...

Image 3 of 5

A tanker passes the yacht as it goes out to sea. A 72 foot Oracle vessel training for the America's Cup, capsized in San Francisco Bay Tuesday October 16, 2012 and currents carried it out into the ocean.

Photo: Brant Ward, The Chronicle

A tanker passes the yacht as it goes out to sea. A 72 foot Oracle...

Image 4 of 5

A 72 foot Oracle vessel training for the America's Cup, capsized in San Francisco Bay Tuesday October 16, 2012 and currents carried it out into the ocean.

The towering Oracle catamaran designed to "fly on water" in its defense of the America's Cup drifted out to sea Tuesday after a spectacular flip into San Francisco Bay during a training run.

Onlookers watched in amazement as the $8 million AC72 racing craft dug its bow into the water, heaved and slammed sideways into the bay shortly after 3 p.m. just off the shoreline near the St. Francis Yacht Club.

The boat quickly drifted to the Marin County side of the bay, prompting crews to try to tow it back to San Francisco. But a mean current caught it and propelled it under the Golden Gate Bridge and into the Pacific Ocean.

"It was amazing - we watched it tip right over, and it looked like the top of the wing came right off," said Ethan Pawson, a UC Berkeley junior who was at the yacht club for a campus regatta ceremony. "Then the big ebb tide just took it right out under the bridge, and it was obvious there was nothing they could do."

U.S. Coast Guard Lt. Joshua Dykman said no one was hurt in the incident and that the 11-member crew did not require outside rescue assistance.

Strongest tide of year

The Oracle team, headquartered at Pier 80 south of the Bay Bridge, dispatched nine safety boats to try to right the craft or tow it back. But the current, driven by the strongest tide of the year, was too much, and the helpless Oracle crews could only accompany the vessel as it was swept 4 miles west of the Golden Gate.

Tom Slingsby, the tactician of Oracle Team USA 17 - the 72-foot racing yacht - said the flip occurred in stiff winds of about 24 knots as he and the rest of the crew were "pushing the limits of the boat." Tuesday marked the eighth time the crew had taken the craft onto the water since its August launch.

"It was pretty scary, I guess," the matter-of-fact Australian, who won a gold medal in sailing at the London Olympics, said at Pier 80 after being brought back from the wreckage by boat. "We've been pushing the boat harder every day, and I guess we found our limit today."

Slingsby said the wing - the long, mast-like apparatus at the center that looks like the wing of an airplane and catches the wind - was destroyed.

"We didn't really know what to expect with the new boat," he said. "When the nose went into the water, most of us hung on and then jumped off. Everyone has a few bumps and bruises, but nothing serious."

Oracle team spokeswoman Lisa Ramsperger said crews would attempt to tow the boat to Pier 80 for repairs. That task was expected to last well into the night and possibly into Wednesday.

Ramsperger said she could not estimate how long repairs will take or the cost.

On the AC72's first voyage, the centerboard broke off and the boat was idled two weeks for repairs.

The boat was skippered by Jimmy Spithill, the Australian who brought the America's Cup to San Francisco by leading BMW Oracle Racing over Alinghi of Switzerland in 2010 in Valencia, Spain.

Racing fans on the shore, who were excited to watch the huge catamaran make a rare appearance, were left stunned after it crashed head over heels. The boat, which can travel twice the wind speed and can top 50 knots, has a mast height of 131 feet.

During the crash, pieces of the enormous wing and rigging snapped off. When the craft drifted beyond the mouth of the Golden Gate around 5 p.m., it was still on its side, but then as the mast broke off, the boat turned upside down.

Good practice for crew

Tom Lyons, a 50-year-old boat racer visiting from Reno, also saw the big splash and said the capsizing, which is not uncommon for racing teams, was unfortunate - but might actually wind up being good practice for the crew.

"This is exactly what they need to get ready for the race," he said.

Slingsby somewhat echoed that sentiment, saying with a grin that he hopes the competition notices how hard the crew pushed its limits. "It might make them wary," he said.

"My coaches always say you learn a lot more from your losses than you do from your victories, and I think we learned some things today."

Stephen Barclay, the CEO of the America's Cup Event Authority, said the only bright side he could see in the accident was that it happened long in advance of the actual Cup races, which are still about a year away. But even that is small comfort, he said.

"The issue for Oracle now is that their boat will be out of action for some time while they do repairs," he said. And that, he said, will adversely affect their training and their assessment of the craft.

Barclay noted that Oracle is preparing a second AC72 boat, but it won't be ready until February.

On the Marin Headlands, dozens of onlookers gathered with binoculars to watch the boat continue out to sea while Oracle's support boats vainly attempted to right it or pull it back under the bridge.

As night fell, the support boats were still trying to figure out the best way to tie onto the boat and haul it back toward land.

Current in control

Chris Raney, a sailor who said he was friends with Oracle crew member John Kostecki, said a boat crash in the bay several years ago humbled him.

"When everything is working OK, you feel invulnerable," Raney said. "Then all of the sudden the mast breaks and you realize you're just on this little stick of fiberglass and the current is going to take you wherever it wants to."

It was the second time an Oracle catamaran pitch-poled on the bay in two weeks. A smaller, 45-foot boat flipped during the America's Cup World Series on Oct. 6, but finished second in the next race and beat Emirates Team New Zealand in the match racing final.

Shortly before Tuesday's mishap, fellow sailors and crew members had posted photos of the boat on Twitter and Instagram with the Golden Gate Bridge as a backdrop. The author of one Tweet noted, "Super windy today - blowing dogs off chains."